Modified port locking not to use system lock for each attempt.
Changed Sg_Ucs4WhiteSpaceP not to handle U+180E (since Unicode 6.3.0 it's not Zs anymore).
Changed intraline check (U+180E is Cf now).
Changed the comment place.
Added Sg_ReadLine so that the get-line and read-line implementations won't be scattered.
Make I/O operation a bit safer.